476  
FXUS02 KWBC 161559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1158 AM EDT MON OCT 16 2017  
 
VALID 12Z THU OCT 19 2017 - 12Z MON OCT 23 2017  
   
..OVERVIEW
 
 
A COOL AND STORMY PATTERN FOR THE NORTHERN HALF OF THE WEST WILL  
GIVE WAY TO DRIER CONDITIONS EARLY NEXT WEEK AS A RIDGE ALOFT  
BUILDS OVER THE WEST, WHILE EASTWARD PROGRESSION OF INITIAL  
EASTERN PACIFIC/WESTERN U.S. ENERGY WILL INCREASE RAINFALL  
POTENTIAL OVER THE CENTRAL U.S./GULF COAST STATES BY NEXT SUN-MON.  
D+8 MULTI-DAY MEANS SHOW GOOD AGREEMENT THAT THE OVERALL PATTERN  
WILL SETTLE INTO AN AMPLIFIED EAST-CENTRAL PACIFIC TROUGH/WESTERN  
U.S. RIDGE/EASTERN TROUGH BY NEXT WEEK. THE LARGE SCALE OF THIS  
EXPECTED PATTERN SHOULD TYPICALLY LEAD TO ABOVE AVERAGE FORECAST  
PREDICTABILITY THOUGH AS THIS FLOW IS IN THE EARLY STAGES OF  
BECOMING ESTABLISHED THERE ARE SOME MEDIUM-SMALLER SCALE DETAIL  
UNCERTAINTIES OVER THE CENTRAL/SOUTHERN AREAS OF THE LOWER 48 BY  
SUN-MON.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
FOR DAYS 3-5 THU-SAT GUIDANCE IS IN VERY GOOD AGREEMENT FOR  
INDIVIDUAL FEATURES ALOFT. A FAIRLY VIGOROUS EASTERN PACIFIC  
TROUGH WILL COME ASHORE EARLY FRI AND REACH THE ROCKIES/HIGH  
PLAINS BY SAT WHILE A RIDGE CROSSES THE EAST. ALSO EARLY IN THE  
WEEKEND AN UPSTREAM MID LEVEL JET WILL BE AIMED AT WESTERN CANADA  
AND THE NORTHERN PACIFIC NORTHWEST, SUPPORTING ANOTHER EPISODE OF  
ENHANCED PRECIP. FOR THIS PERIOD A BLEND OF THE 06Z GFS/00Z ECMWF  
AND TO A LESSER EXTENT THEIR ENSEMBLE MEANS REPRESENTED THE  
CURRENT CONSENSUS QUITE WELL.  
 
AS TROUGHING MOVES INTO THE CENTRAL U.S. AHEAD OF THE BUILDING  
WESTERN RIDGE, MODELS AND ENSEMBLES DIVERGE OVER THE EXTENT AND  
LOCATION OF POTENTIAL FLOW SEPARATION DAYS 6-7 SUN-MON. BEYOND  
THE CURRENT SPREAD IN GUIDANCE THERE HAVE ALSO BEEN SIGNIFICANT  
TRENDS OVER RECENT DAYS. THE AVERAGE OF ALL GUIDANCE HAS  
GENERALLY SHIFTED A BIT EASTWARD WITH THE OVERALL TROUGH. TWO  
DAYS AGO MANY ECMWF-BASED ENSEMBLE MEMBERS/MODEL RUNS WANTED TO  
PULL OFF ENERGY OVER OR JUST WEST OF THE SOUTHERN ROCKIES AS OF  
SUN WHILE THE GEFS/GFS RUNS WERE FLATTER AND PROGRESSIVE. THE TWO  
GUIDANCE SYSTEMS TRENDED TOWARD EACH OTHER YESTERDAY AND NOW  
SOMEWHAT MORE ECMWF MEMBERS OUTPACE GEFS MEMBERS. HOWEVER  
DIFFERENCES ARE MORE PRONOUNCED IN THE 00Z-06Z OPERATIONAL MODELS  
WITH THE 00Z ECMWF STRAYING AHEAD OF ITS MEAN AND THE 06Z GFS  
CLOSING OFF ITS UPPER LOW IN THE SOUTHWESTERN PART OF THE GEFS  
MEAN TROUGH. TELECONNECTIONS RELATIVE TO UPSTREAM FLOW APPEAR TO  
SUPPORT THE ENSEMBLE MEANS THAT ARE MORE MODEST IN THEIR FLOW  
SEPARATION. THIS CONSIDERATION ALONG WITH THE DESIRE TO MAINTAIN  
BETTER CONTINUITY GIVEN CURRENT MODEL/ENSEMBLE SPREAD, LED TO  
PREFERRING INCREASED WEIGHT ON THE 06Z GEFS/00Z ECMWF MEANS  
ESPECIALLY FROM LATE SUN ONWARD.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
THE NORTHWEST U.S. AND NORTHERN CA WILL SEE A RATHER STORMY/WET  
PERIOD IN THE LATE WEEK-WEEKEND TIME FRAME. EXPECT HIGHEST PRECIP  
TOTALS FOR THE PERIOD OVER FAVORED TERRAIN ALONG THE WA/OR COAST  
AND WINDWARD FACING SLOPES OF THE CASCADES. THERE SHOULD BE  
ANOTHER RELATIVE MAXIMUM OVER THE NORTHERN ROCKIES. SYNOPTIC  
SCALE ASCENT IN THE VICINITY OF THE UPPER JET, A SERIES OF  
EMBEDDED SHORTWAVES TAPPING AVAILABLE PACIFIC MOISTURE, AND  
TOPOGRAPHIC LIFT WILL COMBINE TO BRING HEAVY/INTENSE PRECIP AT  
TIMES TO AFOREMENTIONED FAVORED AREAS. INCOMING COLDER AIR, WITH  
A DAY OR SO OF HIGHS AT LEAST 10F BELOW NORMAL AT SOME LOCATIONS,  
WILL SUPPORT LOWERING SNOW LEVELS OVER MOUNTAIN AREAS. THE UPPER  
RIDGE BUILDING OVER THE WEST LATE IN THE PERIOD WILL LEAD TO A  
LIGHTER/NORTHWARD TREND FOR PRECIP THOUGH SOME ACTIVITY COULD  
LINGER OVER EXTREME NORTHERN LOCATIONS, ALONG WITH WARMER TEMPS.  
 
INCREASING MOISTURE RETURN AHEAD OF THE COLD FRONT REACHING THE  
PLAINS BY SAT WILL LEAD TO AN EXPANDING AREA OF LOCALLY HEAVY  
RAINFALL MOST LIKELY OVER THE CENTRAL-SOUTHERN PLAINS DURING THE  
WEEKEND. SOME ENHANCED PRECIP MAY EXTEND FARTHER NORTH ALONG THE  
FRONT AS WELL. BY NEXT MON A FAIRLY LARGE AREA ENCOMPASSING THE  
SOUTHEAST PLAINS TO SOUTHEAST HAS POTENTIAL FOR SOME HEAVY  
RAINFALL BUT WITH LOW CONFIDENCE IN SUCH ACTIVITY AT ANY  
PARTICULAR LOCATION DUE TO WIDENING SPREAD FOR FLOW ALOFT AND  
CORRESPONDING SURFACE PATTERN. EARLIER IN THE PERIOD, LOCATIONS  
ALONG THE WESTERN GULF COAST MAY SEE AREAS OF RAIN WITH AN  
INVERTED SURFACE TROUGH.  
 
MUCH OF THE EAST SHOULD SEE DRY WEATHER FOR MOST OF THE PERIOD  
WITH HIGH PRESSURE DOMINATING. TO THE SOUTH OF THIS HIGH, MOIST  
EASTERLY FLOW MAY BRING PERIODS OF RAIN TO THE FL PENINSULA.  
 
WITH AN UPPER RIDGE CROSSING THE EAST AND WARM FLOW AHEAD OF THE  
FRONT INITIALLY MOVING INTO THE NORTHWEST, THERE WILL BE A BROAD  
AREA OF ABOVE TO MUCH ABOVE NORMAL TEMPS OVER THE LOWER 48 AS OF  
THU WITH STEADY EROSION FROM THE WEST AS THE COLD FRONT PROGRESSES  
EASTWARD. AS FOR GREATEST ANOMALIES, THE NORTHERN PLAINS MAY SEE  
HIGHS OF AT LEAST 15-20F ABOVE NORMAL THU-FRI WHILE PLUS 10-20F  
ANOMALIES EXTEND ACROSS THE UPPER MS VALLEY INTO THE GREAT  
LAKES/NORTHEAST FOR MULTIPLE DAYS. MANY AREAS WILL SEE PLUS  
10-20F ANOMALIES FOR MIN TEMPS AS WELL WITH DEPARTURES FROM NORMAL  
EXCEEDING PLUS 20F MOST LIKELY OVER THE UPPER MIDWEST ON SAT.  
 
RAUSCH  
 

 
 
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